Re: BGP Path Selection weirdness regarding next hops

Ah! I misunderstood. I'll take off my current route maps and add one to
change the MED of the default route.

On Fri, Nov 30, 2012 at 8:38 PM, Marko Milivojevic <markom_at_ipexpert.com>wrote:

> Ah, but when you do that, you're subject to MED comparisons, which
> have their own set of rules. I was referring to the MED on the
> 0.0.0.0/0
>
> --
> Marko Milivojevic - CCIE #18427 (SP R&S)
> Senior CCIE Instructor - IPexpert
>
> On Fri, Nov 30, 2012 at 7:12 PM, John Neiberger <jneiberger_at_gmail.com>
> wrote:
> > No, I increased the MED of the prefix in question, 100.100.100.0/24, in
> my
> > case. The BGP-learned default route is staying at 0 MED.
> >
> > It seems weird to me, too!
> >
> >
> > On Fri, Nov 30, 2012 at 8:07 PM, Yuri Bank <yuribank_at_gmail.com> wrote:
> >>
> >> So you increased the MED of the default route you're receiving? I find
> it
> >> interesting that its the actual metric of each protocol being compared,
> >> regardless of the prefix-length or AD.
> >>
> >> -Yuri
> >>
> >>
> >> On Fri, Nov 30, 2012 at 7:02 PM, Marko Milivojevic <markom_at_ipexpert.com
> >
> >> wrote:
> >>>
> >>> I knew it was a good guess. That's one of my favorites with BGP. It
> >>> gets people unawares all the time :-).
> >>>
> >>> Now, I think Cisco is well within their rights not to touch that part
> >>> of the documentation. The next-hop is *usually* reachable via IGP.
> >>> There are very rare circumstances when the next-hop is reachable via
> >>> BGP *and* is valid for more than hold-down. It seems like you hit one
> >>> of those :-)
> >>>
> >>> Fun.
> >>>
> >>> --
> >>> Marko Milivojevic - CCIE #18427 (SP R&S)
> >>> Senior CCIE Instructor - IPexpert
> >>>
> >>> On Fri, Nov 30, 2012 at 6:55 PM, John Neiberger <jneiberger_at_gmail.com>
> >>> wrote:
> >>> > You are correct! I just did a test by creating a route map to bump up
> >>> > the
> >>> > MED of the prefix in question and it changed the behavior. That
> proved
> >>> > that
> >>> > even though one path now doesn't have an IGP metric to compare, it's
> >>> > still
> >>> > being compared. Maybe Cisco needs to change their documentation to
> say
> >>> > that
> >>> > one of the steps is to compare the metrics, not just "IGP metrics".
> >>> > :-)
> >>> >
> >>> > Thanks!
> >>> > John
> >>> >
> >>> >
> >>> > On Fri, Nov 30, 2012 at 7:37 PM, Marko Milivojevic
> >>> > <markom_at_ipexpert.com>
> >>> > wrote:
> >>> >>
> >>> >> Without going any deeper (some topology information is missing and m
> >>> >> pod is otherwise busy to try this, no matter how FUN it sounds), I'd
> >>> >> venture a guess that yes, "igp" metric is compared.
> >>> >>
> >>> >> The "igp metric" in this sense is really "the metric to reach the
> >>> >> protocol, no matter what that protocol might be". In your case, one
> of
> >>> >> these protocols happens to be BGP. You may want to test this
> hypotesis
> >>> >> by tweaking the BGP's MED value for the default route to make it
> >>> >> numerically higher than OSPF cost to reach the next-hop of the other
> >>> >> route.
> >>> >>
> >>> >> Funnily enough, this is one of the few places where numerical metric
> >>> >> values of different protocols are directly compared, regardless of
> the
> >>> >> AD and/or longest-match.
> >>> >>
> >>> >> --
> >>> >> Marko Milivojevic - CCIE #18427 (SP R&S)
> >>> >> Senior CCIE Instructor - IPexpert
> >>> >>
> >>> >> On Fri, Nov 30, 2012 at 6:21 PM, John Neiberger <
> jneiberger_at_gmail.com>
> >>> >> wrote:
> >>> >> > I posted this question to the Cisco NSP list and I've also talked
> to
> >>> >> > a
> >>> >> > couple of guys from Cisco Advanced Services and I'm still stumped
> >>> >> > about
> >>> >> > something. I'll try my best to phrase it in a way that makes
> sense.
> >>> >> >
> >>> >> > Router A is learning about a prefix from two route reflector
> >>> >> > clients. In
> >>> >> > both cases, the next hop for the prefix is the loopback address of
> >>> >> > the
> >>> >> > advertising routers. Their loopback addresses are being advertised
> >>> >> > into
> >>> >> > OSPF.
> >>> >> >
> >>> >> > So, from the perspective of Router A, it's BGP table for this
> prefix
> >>> >> > has
> >>> >> > two paths:
> >>> >> >
> >>> >> > 1: 4.4.4.4 (loopback address of Router B, learned via OSPF) *
> >>> >> > winner
> >>> >> > due
> >>> >> > to lower IGP metric
> >>> >> > 2. 5.5.5.5 (loopback address of Router C, learned via OSPF)
> >>> >> >
> >>> >> > Now for the weirdness to begin. A network event occurs that causes
> >>> >> > the
> >>> >> > loopback address of Router C to go away. This shouldn't affect
> >>> >> > Router A
> >>> >> > because it is already selecting the shortest path to the network
> via
> >>> >> > Router
> >>> >> > B (4.4.4.4).
> >>> >> >
> >>> >> > However, Router A is also learning a default via BGP. That means
> >>> >> > that
> >>> >> > even
> >>> >> > though 5.5.5.5 (loopback of Router C) disappeared and is
> >>> >> > unreachable,
> >>> >> > the
> >>> >> > router is doing a recursive lookup and keeps the path in the BGP
> >>> >> > table;
> >>> >> > 5.5.5.5 is still reachable, it thinks, by using the default route.
> >>> >> >
> >>> >> > The weird thing is that this causes Router A to start using the
> >>> >> > wrong
> >>> >> > path!
> >>> >> > It seems to be preferring a path with a next hop learned via BGP
> to
> >>> >> > a
> >>> >> > path
> >>> >> > with a next hop learned via OSPF. Why would it do this? I see no
> >>> >> > documentation that would explain why a BGP-learned next hop is
> >>> >> > preferred
> >>> >> > over an IGP-learned next hop.
> >>> >> >
> >>> >> > Is the router still comparing IGP metrics even though the "wrong"
> >>> >> > path
> >>> >> > now
> >>> >> > has no IGP metric?
> >>> >> >
> >>> >> > It's not changing due to router ID, cluster length, or neighbor IP
> >>> >> > address.
> >>> >> > I checked. So, why is it switching?
> >>> >> >
> >>> >> > As soon as the BGP session from Router A to Router C times out,
> the
> >>> >> > extraneous path gets removed from the BGP table and the router
> goes
> >>> >> > back
> >>> >> > to
> >>> >> > using the correct path it should have been using all along.
> >>> >> >
> >>> >> > So, is a BGP-learned next hop preferred over an IGP-learned next
> >>> >> > hop? If
> >>> >> > so, why? If not, any idea why my router switches paths? I've
> turned
> >>> >> > on
> >>> >> > BGP
> >>> >> > debugging and IP routing debugging and haven't found a suitable
> >>> >> > explanation
> >>> >> > for the switch.
> >>> >> >
> >>> >> > John
> >>> >> >
> >>> >> >
> >>> >> > Blogs and organic groups at http://www.ccie.net
> >>> >> >
> >>> >> >
> >>> >> >
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Received on Fri Nov 30 2012 - 20:49:36 ART